1. Field of the Invention
The present invention relates generally to a three prism color separator, and more particularly pertains to a color separator using three joined prisms for color separation in projection displays. The configuration is compact, relatively simple to assemble, and eliminates a requirement for complicated optical components such as crossed dichroic beam splitters and large retrofocus projection lenses. The configuration is particularly advantageous for projectors based upon liquid crystal, reflective light valves.
2. Discussion of the Prior Art
Projection displays based upon reflective liquid crystal light valves (spatial light modulators) generally comprise dichroic beam splitters necessary for color separation and a polarizing beam splitter necessary for light valve operation, which are positioned between the light valves and the projection lens.
In projection displays such as optical monitors, an image from an active device, such as a spatial light modulator, is projected onto a screen at a desired magnification. As the size of the active device is decreased, a reflecting spatial light modulator (SLM) is required to obtain good contrast from the system. For color images, images in three primary colors obtained from three spatial light modulators are simultaneously projected and superposed onto the screen. The optics used to generate color images in this manner typically comprise a dichroic beam splitter, which is a color separation/combining prism set configured as a crossed dichroic cube, and a polarizing beam splitting cube.
The color separation/combining prism set and the polarizing beam splitting cube are positioned between the projection lens and the spatial light modulators, and the optical path through this assembly winds up being at least 150 mm for a 35 mm.times.35 mm spatial light modulator, and may be as large as 200 mm depending upon the geometry of the optics.
In addition, the spatial light modulators work most efficiently with light being incident thereon at substantially normal incidence over a finite conical angle. The polarizing beam splitting cube must operate over the full visible spectrum and over this finite conical angle. This also constrains the projection lens to operate with light reflecting from the spatial light modulators in essentially a normal incidence mode, which amounts to having a projection lens which is designed for a telecentric mode on the SLM side and for a rather large working distance in glass.
Such projection lenses are quite expensive, and may not be very practical because the required asymmetry results in a significantly higher amount of residual lateral color, resulting from a lack of convergence of the three primary color images, which varies linearly with position in the field. Essentially, the three different wavelengths of the three primary colors have slightly different magnifications resulting in chromatic aberration and an imperfect overlap at the edges of the images.
In such prior art projection displays, the two main limitations on the optical quality of the display image are the projection lens and the crossed dichroic cube. The difficulty in the projection lens is in the required long working distance (retrofocus), which must be long enough to encompass the polarizing and color separation/recombination optics. The complexity of design and fabrication of the projection lens and the degradation of its optical performance increases with the length of the working distance.
The optical performance of the crossed dichroic cube must be very high quality for high resolution imaging. The crossed dichroic cube typically consists of four subcomponents assembled together in one cubic unit. Two of the three optical paths in this cube involve imaging the light valves through a split mirror without introducing aberrations. The optical and mechanical tolerances required for assembly of the four subcomponents make the cost of fabrication of the crossed dichroic cube prohibitive for many applications.
Although the crossed dichroic cube is very difficult and costly to manufacture, it provides the most compact optical configuration. Replacing the crossed dichroic cube with individual plate beam splitters or single-color beam splitter cubes only increases the working distance of the projection lens. The additional optics between the lens and the light valves makes the projection lens complicated and costly, and may degrade its performance. The requirement for a large working distance is a particular problem for relatively compact desk top displays since they require lenses with very short lens-to-screen distances.
A typical prior art camera prism assembly is difficult to manufacture as it requires three odd shaped prisms, with an air space being required between the first and second prisms. An air gap can introduce optical distortion, and in order to avoid optical distortion (astigmatism) in high resolution displays, the air space should be uniform and &lt;10 .mu.m.